Opal is a noncrystalline hydrated form of silica, forming when silica slowly settles out of a dilute water solution. Fire opals are transparent to semi-transparent, resembling gelatin, with red, orange, or yellow body color, with or without play-of-color. The body color is caused by inclusions of iron oxides. They are also sometimes called Mexican opals because most of the best fire opals are found in Mexico.